Patients undergo collection of tumor tissue samples at the time of diagnosis, prior to treatment by endoscopic ultrasound or laparoscopy, or during surgical resection following neoadjuvant therapy. Paraffin-embedded tumor tissue specimens obtained at baseline are analyzed by immunohistochemistry to assess tumor vascularity and angiogenic activity. Tumor vascularity is assessed via immunostaining of tumor specimens with the pan-endothelial cell marker, anti-CD34, for evaluation of tumor blood vessels. Angiogenic activity is assessed by analyzing pERK1/2, Ki67, and the pericyte coverage index in tumor specimens. Patients also undergo blood collection to determine plasma levels of VEGF at 4 weeks prior to initial chemotherapy and bevacizumab, at up to 48 hours prior to chemoradiotherapy and bevacizumab, and at up to 48 hours prior to adjuvant chemotherapy and bevacizumab.